drm/i915/dp: fix DG2 max source rate check

Accidentally dropped the else in a rebase fail, causing the DG2 max rate
to be overwritten later in the if ladder.

Fixes: e752d1f9 ("drm/i915/dg2: add DG2 UHBR source rates")
Cc: Manasi Navare <manasi.d.navare@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Manasi Navare <manasi.d.navare@intel.com>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210901112815.16498-1-jani.nikula@intel.com